Just months after completion of a $56 million upgrade of the Moscone Convention Center, the city is moving ahead with plans for a half-billion-dollar expansion of the facility, with an opening date of 2018.

It's a matter of staying competitive in the dog-eat-dog struggle to bring conventions and their hundreds of thousands of big-spending vendors and attendees into the city, said John Noguchi, director of convention facilities for the city.

"This is a huge deal," he said. "There's a lot of competition for convention business throughout the country and it's important that we expand to keep our share of it."

While there was plenty of controversy when the convention center was on the drawing board in the 1970s, much of that disappeared as it became evident just how important the convention business was to the city's economy.

"Tourism is our city's No. 1 industry and bringing Moscone into the 21st century will boost our city's economy," Mayor Ed Lee said in June when he announced the master plan for the Moscone expansion.

Even the typically fractious Board of Supervisors gave unanimous support to the project in a preliminary vote in November.

Tourism brings more than $8.4 billion a year into the city, with a major piece of that coming from the various conventions and trade shows centered on the SoMa convention center. In 2011, more than 650,000 convention-goers visited the city, pouring about $1.8 billion into San Francisco's economy, according to a study by San Francisco Travel, which represents the city's tourism industry.

Outgrowing S.F.

But some of the city's biggest conventions already have outgrown Moscone, said Joe D'Alessandro, the association's president and chief executive officer.

"San Francisco is a desirable location and Moscone is absolutely popular," he said. "But we're starting to hear from a couple of our biggest groups, like the College of Orthopedic Surgeons, that they won't come back unless we expand."

The city already has lost meetings that would have brought in more than $2 billion in spending because of space issues, he said, with cities like Los Angeles, San Diego and Las Vegas already poised to provide a home for the largest business shows and conventions.

Moscone Center has been under nearly continuous construction since the city's voters approved Proposition S in 1976, which cleared the way for a convention center in the Yerba Buena neighborhood.

Moscone South opened in 1981, Moscone North, with its underground connection to the original facility, in 1992 and the stand-alone Moscone West in 2003. The renovation of the two oldest buildings was completed in May.

The expansion plans have been under discussion since 2009, with the city working to put together both a blueprint for the project and a way to pay for it.

"Right now we're in the conceptual design phase, which we hope to have wrapped up in the next couple of months," Noguchi said. "Then we want to launch the (environmental impact report), with the intent to begin construction in December 2014 and open it in spring 2018."

Plans call for construction of a multi-story building near Third and Howard streets, retaining the existing Esplanade Ballroom but adding meeting rooms and office space. The city also would widen the underground concourse beneath Howard Street that connects the two original buildings, changing the two separate exhibition halls into a single, much larger, space.

The wide driveways along both sides of Howard Street would disappear, with building lobbies extending to the sidewalk.

"We'd also like to build up on Moscone South, but we have to be conscious of the restaurants and open space already there," Noguchi said.

One of the most important concerns of the expansion is to better connect the convention center to the surrounding neighborhood, which could mean the addition of stores and restaurants.

"Our top priority in the design is to improve the existing street scene," he added, which means that Yerba Buena Gardens, the carousel and other local amenities won't be changed by the construction.

As for financing, the supervisors have given preliminary approval to a plan that pays for much of the project with money from tourist hotels. Hotels that together would pay at least half of the $19.3 million first-year assessment would have to approve the proposed Moscone Expansion District in a mail-only election that ends Feb. 6.

The city won't get off free, though. The plan calls for $8.2 million in general fund payments for the expansion effort the first year, rising to an annual maximum of $10.7 million over the 32-year term of the project.

Always looking ahead

Paying off the necessary bonds is a long-term proposition, but the convention business is one that is always looking way down the road, said D'Alessandro.

"As soon as we break ground for the expansion, we can begin selling the larger facility," he said. "We're already booking conventions into 2030."